This invention relates to a method and means for rendering safe the opening or removal of a machine door, cover or guard which is normally kept locked to guard machinery or an area which is potentially hazardous.
British Pat. No. 1,260,322 discloses a safety device comprising a lock unit for connection to a fixed structure of machinery, and a captive key unit for connection to a door, cover or guard of the machinery, the two units being adapted so that they can be engaged and locked together by rotation of the captive key unit when the door is closed. The lock unit also incorporates electrical switching means which is controlled by rotation of the captive key unit and which in turn controls the power supply to the machinery so that power is connected only when the two units are locked together and is disconnected when the captive key unit is turned to an open position in which the two units can be freely disengaged and engaged to open and close the door.
This known safety device is adequate for machinery which is rendered safe as soon as the power supply is disconnected. However, it is not adequate for machinery which remains potentially dangerous for some time after power is disconnected, because the door can then be opened while the machinery is still in a dangerous state. For example, the machinery may have rotating parts which do not come to rest immediately power is disconnected, either because of the inertia of these parts or because they are stopped gradually, such as in a textile mill to prevent the breaking of fibres. An object of the present invention is to provide a safety device which overcomes this shortcoming of the known safety device.